Thick film materials are known in the art, which are mixtures of metal, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films are used in a wide variety of electronic and light electrical components.
The properties of such thick film compositions depend on the specific constituents of the compositions. Most of such thick film compositions contain three major components. A conductive phase determines the electrical properties and influences the mechanical properties of the final film. A binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to a substrate and an organic medium (vehicle) acts as a dispersing medium and influences the application characteristics of the composition and particularly its rheology.
High stability and low process sensitivity are critical requirements for thick film resistors in microcircuit applications. In particular, it is necessary that resistivity (Rav) of a resistor be stable over a wide range of temperature conditions. Thus, the temperature coefficient of resistance (TCR) is a critical variable in any thick film resistor. Because thick film resistor compositions are comprised of a functional (conductive) phase and a permanent binder phase, the properties of the conductive and the binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Heretofore, thick film resistor compositions, and especially the air fired ones, have been formulated with cadmium and lead bearing glasses and Ru-based conducting materials. Some of the Ru-based materials are RuO2—see U.S. Pat. Nos. 3,868,334 and 4,101,708—or lead ruthenate Pb2Ru2O6+δ—see U.S. Pat. No. 3,682,840. A family of pyrochlores such as Bi2Ru2O7 and many other conducting oxides derived from Bi2Ru2O7 are described in U.S. Pat. Nos. 3,583,931 and 3,681,262. The use of these pyrochlores in thick film resistors is described in U.S. Pat. Nos. 3,560,410, 3,553,103 and 3,630,969. Lead and cadmium containing glasses used in thick film resistors have been disclosed by numerous U.S. Patents. U.S. Pat. No. 5,753,571 discloses lead and cadmium free glass compositions, containing specified amounts of bismuth oxide, silica and other oxides, for encapsulating electronic hybrid circuits. U.S. Pat. No. 5,439,852 discloses lead and cadmium free thick film compositions comprising electroconductive particles.
The air fired thick film resistors compositions of the prior art have good properties. However, they have a number of shortcomings: e.g., cadmium containing materials are known carcinogens and lead compounds are highly toxic. Most producers of thick film resistors have addressed the cadmium issue and have developed new Cd-free compositions.
Elimination of lead from thick film resistor composition is more difficult and complex. The difficulty is probably due to the major proportions of lead oxide in the conductive phase and in the glasses. Lead containing glasses have a unique combination of properties such as expansion, viscosity, durability and surface tension, which make them very desirable for thick film resistor compositions. Furthermore, when conductive materials such RuO2, Bi2Ru2O7 and substituted bismuth ruthenate are formulated with Cd- and Pb-free glasses, the resistance range is too small for RuO2 based resistors and the bismuth ruthenate-type conductives interact with these glasses in such a way that partial to complete leaching of Bi2O3 occurs. This interaction may be described by: Bi2Ru2O7+Cd- and Pb-free glass→RuO2+Bi-glass. The interaction of bismuth ruthenate and substituted bismuth ruthenate compounds with leadless glasses limits the resistance range and does not allow control of the resistance. The leaching of Bi2O3 from the bismuth ruthenate-type conductive phases into the leadless glass may be prevented by the use of bismuthate glasses. However, bismuth ruthenate and substituted bismuth ruthenates, when formulated with bismuthate glasses, produce negative TCR. U.S. Pat. No. 5,491,118 describes lead-free and cadmium—free thick film resistor compositions. Said patent describes the use of Bi2Ru2O7, BiGdRu2O7 and RuO2 conducting phases with alkaline earth and bismuthate glasses. The bismuthate glasses are described in the aforecited U.S. Pat. No. 5,439,852. Low resistance resistors formulated with RuO2 having good TCR are detailed and higher resistance resistors (˜30 kΩ) with BiGdRu2O7 are also detailed. However, at high resistance the TCR characteristics and especially the TCR gap, i.e., HTCR-CTCR are too large.
Thus, there is a need for new resistor compositions which are Cd and Pb-free and comprise a stable, air fired, conductive phase, whose interactions with the leadless glass are small. Moreover, the required resistance range should be in the range of ˜10 kΩ/□ to ˜mega Ω/□ with good TCR characteristics (Ω/□ depicts surface resistance).